238U Decay Series Systematics Of Young Lavas From Batur Volcano, Sunda Arc

Abstract

Activities of 238U decay series radioisotopes have been determined for both postcaldera basalts erupted between 1849 and 1974 and genetically related young precaldera dacites from Batur volcano, Bali, Sunda arc. All rocks possess (230Th/238U) = 1 within 2 sigma error indicating that little, if any, fractionation between Th and U occurred during their genesis, or in their source regions, within approximately the last 350 ka. Both the basaltic and the dacitic rocks possess (230Th/232U) ~ 0.71, consistent with both having a common mantle source with an unusually high Th/U weight ratio of 4.4 and with the dacitic magma having formed by closed-system convective fractionation of the same basaltic magma from which the postcaldera lavas were derived. The historical basalts possess (226Ra/230Th) > 1, indicating a significant Ra enrichment relative to Th and U within approximately the last 8 ka. The highest degree of 226Ra enrichment occurs in the most mafic basaltic lavas, which form a cycle of mafic volcanism between 1888 and 1926. If Ra enrichments were produced during mantle-melting as a result of its greater incompatibility relative to Th then variations in (236Ra/230Th) may reflect the presence at Batur of a periodically replenished, but poorly mixed, open-system magma chamber. In addition, this result implies that the primitive Batur basaltic magmas take considerably less than about 8 ka to travel from their mantle source through the Batur magma chamber to the surface. Whole rock (210Pb/226Ra) > 1 in the youngest historical basalts indicates that Pb has been enriched relative to Ra within about 100 years of their eruption. Because Pb is expected to be less incompatible then Ra during mantle melting, the enrichments are probably due to late stage processes, such as accumulation of 210Pb-rich plagioclase phenocrysts or post-eruptive condensation of volatile Pb salts in vesicles.